About Chilling Hours,Units & Portions

Stone and pome fruit trees rely on enough chilling for flowers and leaf buds to develop normally. If the buds do not receive sufficient chilling temperatures during winter to completely release dormancy, trees may develop physiological symptoms such as delayed and extended bloom, delayed foliation, reduced fruit set and reduced fruit quality. Growers and industry traditionally keep track of chilling hours beginning in November to get a sense of the orchard management practices needed and comparison of past year's weather and crop load. The approximate number of hours/units/portions needed for normal development varies depending on variety and species. For more information contact local nurseries or farm advisors.

Chilling Hours and Units models: The publication Chilling Accumulation: Its Importance and Estimation (David H. Byrne and Terry Bacon, Dept. Of Horticultural Sciences, Texas A&M University) explains the different methods of accumulating chilling units. There are three common models used to calculate chilling, each one defining what a chilling unit is: 1) Hours below 45°F model; 2) Hours between 32°F and 45°F model; and 3) Utah model.

The Byrne and Bacon explanation of calculations used by the Utah model:

The program calculations for the Utah model accessed on this website:

1 hour below 34°F = 0.0 chill unit

1 hour 35 - 36°F = 0.5 chill units

1 hour 37 - 48°F = 1.0 chill units

1 hour 49 - 54°F = 0.5 chill units

1 hour 55 - 60°F = 0.0 chill units

1 hour 61 - 65°F = -0.5 chill units

1 hour >65°F = -1.0 chill units

1 hour below 34°F = 0.0 chill unit

1 hour 34.01 - 36°F = 0.5 chill unit

1 hour 36.01 - 48°F = 1.0 chill unit

1 hour 48.01 - 54°F = 0.5 chill unit

1 hour 54.01 - 60°F = 0.0 chill unit

1 hour 60.01 - 65°F = -0.5 chill unit

1 hour >65.01°F = -1.0 chill unit

Dynamic Modelor Cumulative Chilling Portion- The ability to calculate chill portions using the dynamic model's calculation method became available in September 2006. The model is set to start each year on Sept 1, based on decisions made due to early chill some years, ongoing research interests, realities of programming, and figuring that inclusion is better than exclusion when it comes to data. The best use of the model for research purposes and for industry applications is a matter of change over time as we learn more about the subject and its applications. While this information is made available as a public service, I hope that all users will be aware that it is more a research tool at this point than policy written in stone with respect to recommended usage. - Kitren Glozer, September 2006